This application is related to commonly assigned, copending U.S. patent application Ser. No. 08/390,732 filed Feb. 17, 1995. Application Ser. No. 08/390,732 is incorporated herein by reference.
The present invention relates to a thermal spray powder. In particular, the invention relates to molybdenum-based thermal spray powders useful for producing wear resistant coatings on the sliding contact friction surfaces of machine parts such as piston rings, cylinder liners, paper mill rolls, and gear boxes.
Thermally sprayed molybdenum coatings, due to their unique tribological properties, are useful in the automotive, aerospace, pulp and paper, and plastics processing industries. Molybdenum coatings provide a low friction surface and resistance to scuffing under sliding contact conditions.
Coatings which are flame sprayed from molybdenum wire sources are widely used in the automotive industry as, e.g., running surfaces on piston rings in internal combustion engines. The high hardness of these coatings is attributable to the formation during spraying of MoO2 which acts as a dispersion strengthener. However, the process of flame spraying coatings from molybdenum wire is not sufficiently versatile for the more complex applications being developed for molybdenum coatings. Some of these applications require higher combustion pressures and temperatures, turbocharging, and increased component durability. The molybdenum wire produced coatings do not meet these requirements. Further, there is an increasing need for the tailoring of coating properties based on periodically changing design requirements. Powder based coating technologies, e.g., plasma powder spray offer flexibility in tailoring material/coating properties through compensational control, which is not readily achievable using wire feedstock.
Coatings which are plasma sprayed from molybdenum powder are more versatile than coatings from wire, but are relatively soft, and do not exhibit adequate breakout and wear resistance for the automotive and other applications described above. The molybdenum tends to oxidize during spraying, leading to weak interfaces among the lamellae of the coating and to delamination wear. Also, the aqueous corrosion characteristics of molybdenum coatings are poor.
The molybdenum powder may be blended with a nickel-based self-fluxing alloy powder, for example, powder including nickel, chromium, iron, boron, and silicon, to form a Mo/NiCrFeBSi dual phase powder (also referred to in the art as a pseudo alloy). The improved wear characteristics of a coating flame sprayed from the blend result in a wear resistant coating with desirable low friction properties and scuff resistance.
When this pseudo-alloy powder blend is plasma sprayed, however, the molybdenum particles and the NiCrFeBSi particles tend to form discrete islands in the coating. Although the overall hardness is greater, in microscopic scale the molybdenum islands are still soft and are prone to breakout and failure. Once the wear process is initiated, the coating exhibits rapid degradation with increased friction coefficient, particle pull out, and delamination.
Another improvement in plasma sprayed molybdenum coatings is described in the publication by S. Sampath et al., xe2x80x9cMicrostructure and Properties of Plasma-Sprayed Moxe2x80x94Mo2C Compositesxe2x80x9d (J. Thermal Spray Technology 3 (3), September 1994, pp. 282-288), the disclosure of which is incorporated herein by reference. A dispersion strengthened coating is plasma sprayed from a Moxe2x80x94Mo2C composite powder. The Mo2C particles dispersed in the molybdenum increase the hardness of the coating. Also, the carbon acts as a sacrificial oxygen getter, reducing the formation of oxide scales between molybdenum lamellae of the coating during spraying and decreasing delamination of the coating. However, the hardness, wear resistance, and aqueous corrosion resistance of the coating is not sufficient for some applications.
Further improvement in plasma sprayed molybdenum coatings is described in above-referenced application Ser. No. 08/390,732. The dual phase powder blend disclosed in application Ser. No. 08/390,732 adds NiCrFeBSi powder to the above-described Moxe2x80x94Mo2C composite powder. The coating made from this powder blend exhibits discrete islands similar to those described above for the Moxe2x80x94NiCrFeBSi coating. The NiCrFeBSi islands have similar advantageous properties to those described above; however, the Mo2C particles dispersed in the molybdenum increase the hardness of the molybdenum islands, slowing degradation of the coating. Also, the carbon acts as a sacrificial oxygen getter, reducing the formation of oxide scales on the molybdenum islands of the coating during spraying and decreasing delamination of the coating, as described above. However, the aqueous corrosion resistance and/or hardness of the coating are still not sufficient for some applications.
The present invention is directed to even further improving the properties of molybdenum coatings, whether they are plasma sprayed or flame sprayed.
Accordingly, it is an object of the present invention to overcome the disadvantages of the prior art molybdenum-based thermal spray powders and coatings.
It is another object of the invention to provide molybdenum-based thermal spray powders, as well as powder blends including such powders, for spraying of improved coatings with high aqueous corrosion resistance, high cohesive strength, and uniform wear characteristics without significant loss of sprayability of the powders or of low friction characteristics of the coatings made therefrom.
It is a further object of the invention to provide high hardness, low- and stable-friction coatings exhibiting high aqueous corrosion resistance, high cohesive strength, and uniform wear characteristics.
Accordingly, in one embodiment the invention is a molybdenum-based composite powder for thermal spray applications, the composite powder including an alloy selected from molybdenum-chromium, molybdenum-tungsten, and molybdenum-tungsten-chromium alloys dispersion strengthened with molybdenum carbide precipitates. In a narrower embodiment, the molybdenum-based composite powder includes about 10-30 weight percent of chromium and/or tungsten, about 1-3 weight percent carbon, remainder molybdenum.
In another embodiment, the invention is a blended powder for thermal spray applications, the blended powder including a mixture of (a) a molybdenum-based alloy selected from molybdenum-chromium, molybdenum-tungsten, and molybdenum-tungsten-chromium alloys dispersion strengthened with molybdenum carbide precipitates, and (b) a nickel-based or cobalt-based alloy. In a narrower embodiment, the blended powder consists essentially of about 10-50 weight percent of the nickel-based or cobalt-based alloy, the remainder being the dispersion strengthened molybdenum-based alloy. In still narrower embodiments, the nickel-based or cobalt-based alloy may be a self-fluxing nickel-based alloy comprising nickel, chromium, iron, boron, and silicon, or a Hastelloy(copyright) (nickel-based) alloy, or a Tribaloy(copyright) (cobalt-based) alloy. (Hastelloy and Tribaloy are registered trademarks of Haynes International and Stoody Deloro Stellite, respectively.)
In a further embodiment, the invention is a thermal spray coating having lamellae of a molybdenum-based alloy selected from molybdenum-chromium, molybdenum-tungsten, and molybdenum-tungsten-chromium alloys dispersion strengthened with molybdenum carbide precipitates. In a narrower embodiment, the thermal spray coating further includes lamellae of a nickel-based or cobalt-based alloy. In still narrower embodiments, the nickel- or cobalt-based alloy may be a self-fluxing nickel-based alloy comprising nickel, chromium, iron, boron, and silicon, or a Hastelloy alloy, or a Tribaloy alloy.
In one exemplary embodiment of the composite powder in accordance with the invention, the properties of a molybdenum-based coating are improved by the addition to the molybdenum of chromium and a small amount of carbon. The chromium forms with the molybdenum a solid solution molybdenum-based alloy, while the carbon reacts with the molybdenum to form molybdenum carbide (Mo2C) precipitates dispersed throughout the molybdenum-chromium alloy to dispersion strengthen the alloy. As used herein, the term xe2x80x9cmolybdenum-basedxe2x80x9d is intended to mean an alloy or composite including at least 50 weight percent total molybdenum (reacted and elemental). The amount of carbon is selected based on the amount of Mo2C desired in the composite powder, which typically is about 20-60 volume percent of the composite powder. Preferably, the dispersion strengthened alloy includes about 10-30 weight percent chromium, about 1-3 weight percent carbon, remainder molybdenum.
The chromium component in the alloy is included to provide improved corrosion resistance over a Moxe2x80x94Mo2C powder, while the presence of the carbide in the composite powder provides some dispersion strengthening. The chromium also provides some additional strengthening to the coating. Oxidation of the carbide during thermal spraying provides an additional benefit in that, during the spraying process, the carbon acts as a sacrificial getter for oxygen, reducing the oxidation of molybdenum. With such gettering, oxide free lamellar surfaces can be produced resulting in improved bonding of the molybdenum-chromium alloy lamellae to one another. Thus, delamination during sliding contact is reduced, resulting in a stable coefficient of friction and improved wear resistance.
In another, similar, molybdenum-based composite powder, the chromium is replaced by tungsten. The tungsten and a small amount of carbon are added to the molybdenum to form a solid solution alloy dispersion strengthened with Mo2C. Again, the amount of carbon is selected based on the amount of Mo2C desired, typically about 20-60 volume percent, in the composite powder. Preferably, the dispersion strengthened alloy includes about 10-30 weight percent tungsten, about 1-3 weight percent carbon, remainder molybdenum.
The alloy of molybdenum and tungsten provides solid solution strengthening to the composite coating, and can provide improved high temperature properties, while the dispersed carbide provides the dispersion strengthening and lamellar bonding benefits described above. The coating exhibits a stable coefficient of friction, improved wear resistance, and high temperature strength.
Alternatively, both chromium and tungsten powders may be added with the carbon powder to the molybdenum powder to form the molybdenum-based alloy. Again, the amount of carbon is selected based on the amount of Mo2C desired in the composite powder. Preferably, the dispersion strengthened alloy coating includes about 10-30 weight percent of a combination of chromium and tungsten, about 1-3 weight percent carbon, remainder molybdenum.
The chromium component in the alloy provides improved corrosion resistance and hardness, the tungsten component provides added hardness and strength, and the carbide contributes some strengthening and the above-described improved bonding of the molybdenum-chromium-tungsten alloy lamellae to one another. The optimum ratios of chromium to tungsten and of chromium or tungsten to molybdenum in the blend to provide the desired strengthening and corrosion resistance for a particular application may be determined empirically.
The molybdenum-based composite powders may be produced, e.g., by a method similar to that described in U.S. Pat. No. 4,716,019 for producing a molybdenum powder dispersion strengthened with molybdenum carbide (Moxe2x80x94Mo2C powder). U.S. Pat. No. 4,716,019 is incorporated herein by reference. The process involves forming a uniform mixture of fine powders of molybdenum and chromium and/or tungsten with a carbon powder having a particle size no greater than that of the metal powders. The amount of the carbon powder is selected based on the amount of molybdenum carbide desired in the composite powder. Alternatively, a molybdenum-chromium or molybdenum-tungsten, or molybdenum-chromium-tungsten alloy may be mixed with the carbon powder. Again, the amount of the carbon powder is proportional to the amount of molybdenum carbide desired in the composite powder.
A slurry is formed from one of these powder mixtures, an organic binder, and water, with the amount of the binder typically being no greater than about 2 weight percent of the powder mixture. The powders are then agglomerated from the slurry, e.g., by spray-drying. Preferably, the agglomerated powders are classified to select the major portion of the agglomerates having a size greater than about 170 mesh and less than about 325 mesh. The selected agglomerates are reacted at a temperature no greater than about 1400xc2x0 C. in a non-carbonaceous vessel in a reducing atmosphere for a time sufficient to form the agglomerated composite powder. The (Mo,Cr)Mo2C, (Mo,W)Mo2C, or (Mo,Cr,W)Mo2C powder thus produced retains the desired sprayability and may be used in plasma or flame spraying processes to produce coatings exhibiting high cohesive strength, high aqueous corrosion resistance, stable coefficient of friction, and uniform wear characteristics.
An even further improved coating may be produced from a dual phase powder blend of one of the above-described molybdenum-based composite powders with a nickel-based or cobalt-based alloy. As used herein, the term xe2x80x9cnickel-basedxe2x80x9d or xe2x80x9ccobalt-basedxe2x80x9d is intended to mean alloys or powder mixtures in which nickel or cobalt, respectively, is the major component. A typical example of such a dual phase powder blend is a mixture of about 50-90 weight percent of the above-described dispersion strengthened molybdenum-tungsten, molybdenum-chromium, or molybdenum-chromium-tungsten alloy with about 10-50 weight percent of a self-fluxing nickel-boron-silicon alloy. The nickel-boron-silicon may include such other components as chromium, iron, and/or carbon. Typical of such alloys are the self-fluxing NiCrFeBSi alloy powders described above. A typical composition for such a self-fluxing alloy is, in percent by weight, 0 to about 20% chromium, 0 to about 4% iron, about 2-5% boron, about 2-5% silicon, 0 to about 2% carbon, remainder nickel. One example of a preferred composition for such a self-fluxing alloy is, in percent by weight, 13.6% chromium, 4.4% iron, 3.3% boron, 4.4% silicon, 0.8% carbon, remainder nickel. The coating exhibits improved sprayability, cohesive strength, hardness and wear resistance over the molybdenum-based composite powder alone and results in a coating showing uniform wear, a low coefficient of friction, and good cohesive strength.
Alternatively, a similar dual phase powder may be made by mixing the above-described dispersion strengthened molybdenum-chromium, molybdenum-tungsten, or molybdenum-chromium-tungsten alloy with a commercially available high temperature, moderate hardness, corrosion resistant nickel-based alloy such as a Hastelloy C or Hastelloy D alloy, or of a commercially available high temperature, high hardness, corrosion resistant cobalt-based alloy such as a Tribaloy alloy. The preferred proportions for such a blend are about 50-90 weight percent of the molybdenum-based alloy and about 10-50 weight percent of nickel- or cobalt-based alloy. The Hastelloy alloy component provides further improvement in the corrosion resistance of the sprayed coating, while the Tribaloy alloy component provides a combination of further improved wear and corrosion resistance. The dual phase powder blend may be tailored to provide a coating of selected hardness, wear resistance, corrosion resistance, coefficient of friction, etc. by selection of the dispersion strengthened molybdenum-based alloy component, the nickel- or cobalt-based alloy component, and their ratio by empirical means.
The above-described blended powders combining the dispersion strengthened molybdenum-based alloy with a nickel- or cobalt-based alloy may be produced by making the dispersion strengthened molybdenum-based alloy powder as described above then blending this powder with a nickel- or cobalt-based alloy powder, in accordance with commercially accepted metal powder blending technology. Typically, the nickel- or cobalt-based alloy powders are produced from the alloys by gas atomization. Alternatively, a commercially available nickel- or cobalt-based alloy powder may be used in the blend.
To form the above-described coatings, the composite or blended powders are thermally sprayed, e.g., by known plasma spraying or flame spraying techniques, onto the bearing or friction surfaces of a metal machine part subject to sliding friction, forming a wear resistant, low-friction surface.